1. Field of the Invention
The present invention relates generally to fluid flow control valves of the type wherein a member is moved to various positions between open and closed positions in order to adjust the flow rate of a fluid flowing through the valve. Specifically, the invention relates to those of such valves which are of the so-called eccentric rotary type, wherein the moved member is a plug which is positioned by being rotated about an axis which is normal to, and is offset from, the axis of the cooperating valve seat. More specifically, the invention relates to valves of the last-noted type in which the rotating and positioning of the plug about its rotational axis is desirably effected by an actuator which must establish a discreet plug position, and corresponding valve opening, for each of a plurality of values of a controlling signal, such as a controlling air pressure, which is applied to the actuator.
2. Description of the Prior Art
In the known valves of the type last identified above with which I am familiar, the plug is carried by a supporting means which, in turn, is secured to a shaft. The latter is rotated by the valve actuator about the above-noted rotational axis in order to move the plug into the desired one of its positions. That position is the one which corresponds to the existing value of the control signal, such as a controlling air pressure, which is applied to the actuator. The intention is that each such control signal value provides a corresponding or unique plug position or plug or valve opening which produces a corresponding, desired flow rate for the fluid passing through the valve. A typical example of such a valve is the valve which is disclosed in the Myers U.S. Pat. No. 3,963,211, and a typical example of such an actuator in the air-operated actuator which is disclosed in the Smith U.S. Pat. No. 3,985,151. Although, in the interest of clarity and simplicity, the invention will be described in connection with a valve which is shown as being actuated by an air-operated actuator, it is to be understood that the invention is applicable as well to valves which are actuated by other types of actuators, or by other means.
With respect to the known valves of the type being considered, it has been found in practice that the actuator loses control of the plug position for plug positions lying in a so-called unstable control region which exists between the valve-closed and valve-open positions of the plug. For convenience, this region will be referred to hereinafter simply as the unstable region. The location of this region in the range of plug movement or travel varies from one valve arrangement to another, and is usually expressed in degrees of plug rotation from the valve-closed position.
When the plug is moved into the unstable region in a given situation, the plug can have more than one position for each value of the controlling air pressure which is applied to the actuator. Consequently, for plug positions in this region, the plug position is no longer uniquely related to the value of the controlling air pressure, and control of the plug position and amount or degree of valve opening are lost. As will be explained more fully hereinafter, it is my understanding that this is due to the bistable nature or characteristic of a rotative force which the flowing fluid causes the plug to exert on its shaft, which force, at a so-called critical position of the plug, passes through a minimum which gives the force its bistable nature with respect to the plug position and establishes the unstable region.
Since control of the plug position by the actuator is not obtainable for plug positions lying in the unstable region, it has been necessary with the known valves to limit the extent or degree of valve opening to that which does not cause the plug to move into the corresponding unstable region. This has amounted to a serious practical shortcoming for the known valves, since it is usually desirable to be able to open and control the opening of a valve up to its full capacity, and since the positions of the unstable regions in the known valves are usually such as to prevent those valves from being used over plug opening ranges which are commensurate with the sizes and hence full capacities of the valves. In short, said shortcoming has prevented the use of the known valves to the full extent of their theoretical capabilities.
It has been proposed in the past to mitigate the effects of the above-noted shortcoming by increasing the spring force which is provided in the actuator in opposition to the controlling air pressure, thereby to shift the unstable region somewhat in the valve-open direction. Such a procedure has the practical disadvantage of requiring a corresponding increase in the force which the actuator must exert in opposition to the spring force, and hence of requiring a corresponding increase in the size and cost of the actuator.
Another shortcoming of the known valves of the type being considered which has prevented the realization of their potential capabilities is their use, in achieving the rotative support of their plugs, of force transmitting or coupling members which appear in flow-impeding positions in the fluid flow path, particularly in the main portion of the fluid flow. An example of such a member is the member 42 of the valve of the above-noted Myers patent. Such a member is undesirable to the extent that it impedes the fluid flow through the valve, and hence reduces the flow capacity of the valve below its otherwise available value.